Expose Test helpers for testing custom locker implementations

[mfridman]

Users implementing custom Locker or SessionLocker interfaces would benefit from access to test helper functions to verify their implementations function correctly under concurrent access.

Currently, these helpers are only available internally, but since they use the public Locker and SessionLocker interfaces, they could be exposed to assist users in testing their custom locker implementations to ensure they follow the expected locking contract (such as only one locker acquiring the lock, others failing appropriately, avoiding deadlocks, etc.). This approach would be similar to how other Go packages expose test helpers for interface implementations.

Consider moving it to a public testing package.